Starting point will be the principle iteself- the right and effective places on the human body from where the signals can be taken to measure.
Possible points for measuring the heart rate are:

The ventral aspect of the wrist on the side of the thumb (radial artery).
The ulnar artery.
The neck (carotid artery).
The inside of the elbow, or under the biceps muscle (brachial artery).
The groin (femoral artery).
Behind the medial malleolus on the feet (posterior tibial artery).
Middle of dorsum of the foot (dorsalis pedis).
Behind the knee (popliteal artery).
Over the abdomen (abdominal aorta).
The chest (apex of the heart), which can be felt with one’s hand or fingers. It is also possible to auscultate the heart using a stethoscope.
The temple (superficial temporal artery).
The lateral edge of the mandible (facial artery).
The side of the head near the ear (posterior auricular artery).